Multi-stage vacuum pump

ABSTRACT

A multi-stage vacuum pump includes a plurality of rotor elements ( 14, 16 ) disposed on a common shaft ( 10 ) in a pump housing ( 12 ) for configuring multiple pump stages ( 18, 24, 26 ). The shaft is driven by an electric motor ( 40 ). An inner bearing element ( 42 ) is disposed between two rotor elements ( 14, 16 ) such that a mechanically favorable bearing arrangement is implemented using a simple configuration of the bearing elements ( 42, 44 ) as roller bearings. This is possible in particular due to the separation into two rotor elements ( 14, 16 ).

The invention relates to a multi-stage vacuum pump.

Multi-stage vacuum pumps are configured e.g. as multi-inlet vacuum pumpshaving at least two inlets and one outlet. Said inlets are connected todifferent vacuum stages of the multi-inlet pump, wherein a differentvacuum is generated at each inlet. Normally, in such an arrangement, thehighest vacuum is generated by the inlet connected to the first stage ofthe multi-inlet pump, the second highest vacuum is generated by theinlet connected to the second stage, and so forth. Such vacuum pumpswith a plurality of vacuum stages comprise, within a housing, a shaftwhich is driven by an electric motor, the latter being normally arrangedaround said shaft.

From the state of the art, there are known various types of bearingarrangements for said shaft. Particularly, it is known to support theshaft via two ball bearings, the electric motor being arranged betweensaid two ball bearings. In the direction of the first stage, the shaftcomprises a shaft extension. On said shaft extension, the rotor elementis arranged. Thus, the rotor is arranged on the projecting shaftextension and accordingly is supported in a cantilevered manner. Sinceall of the forces acting on the rotor will be transmitted to the shaftat the cantilevered end of the shaft, the bearings are subjected to highstresses. Further, in shafts supported in this manner, the length of theshaft is limited because, otherwise, it would not be possible anymore totake up the occurring forces in the bearings, or one would have to useextremely complex and expensive bearings. A further disadvantage of thisbearing arrangement resides in the relatively small bearing spacing.

In another bearing arrangement of a similar type, the electric motor isnot arranged between the two bearings but externally of the bearing.Also here, the rotor is arranged on the cantilevered extension of theshaft and thus again has the disadvantages of a cantilevered bearingarrangement. This will cause an unfavorable position of the center ofgravity and, as a result, high stresses acting on the bearing.

Further, from DE 603 13 493, it is known to support the shaft on both ofits ends. Because of the large bearing spacing resulting from such anarrangement, the forces occurring in the bearings can be equalized andreduced. However, the bearing arranged on the suction side of the pump,i.e. in the region of the first stage, has to be designed as a magneticbearing, which is of necessity due to the low pressure existing in thisregion. According to the state of the art, a grease-lubricated ballbearing is unsuited for use in this region because the low pressurewould then cause the grease to be sucked out from the bearing. Asusually the case in magnetic bearings, there is additionally provided afurther ball bearing serving as a retainer bearing, while this bearing,however, is not used for take-up of forces but only as an emergencybearing. Due to the required provision of a permanent magnetic bearingand additionally of a retainer bearing, this type of bearing arrangementis expensive. Further, it is required to provide a star-shaped holdingelement for the permanent magnetic bearing, said holding elementcomprising flow passage openings. Since this star-shaped bearing shieldis located in the region of the high vacuum stage, conductance losseswill occur on extremely unfavorable sites in the course of the flow.Such occurrences will cause a deterioration of the maximum performanceof the vacuum pump.

It is an object of the invention to provide an inexpensive and effectivebearing arrangement for multi-stage vacuum pumps which particularly alsoallows for a reduction of the constructional length of the pump.

According to the invention, the above object is achieved by the featuresdefined in claim 1.

As provided by the invention, the rotor is divided into at least tworotor elements. Thus, the two rotor elements are arranged separatelyfrom each other, and particularly are connected to the shaft separatelyfrom each other. Herein, depending on the configuration of themulti-stage vacuum pump and particularly on the arrangement of theinlets, one or also a plurality of stages can be provided per rotorelement. By the inventive division of the rotor into two rotor elements,it is possible to arrange an inner bearing element, usually a rollingbearing such as a ball bearing, between the two rotor elements.Preferably, for this reason, one of the two rotor elements, particularlythe rotor element forming or including the high vacuum stage, isarranged externally of the inner bearing element. The rotor element isthus arranged on a shaft extension which is projecting relative to theinner bearing element. Since, however, in contrast to the state of theart, it is not the whole rotor but only one of at least two rotorelements that is arranged on the cantilevered end of the shaft, theforces and moments introduced into the shaft at the cantilevered endthereof will be considerably smaller. The second rotor element can bearranged e.g. between an inner bearing element and an outer bearingelement and particularly be fixedly connected to the shaft.

Since, according to the invention, the inner bearing element ispreferably not arranged in the region of the high vacuum but instead isarranged within the outer rotor element comprising the high vacuumstage, the bearing will not be subjected to the extremely low pressureswhich prevail in the region of the high vacuum. This offers theinventive advantage that especially grease-lubricated bearings such ase.g. ball bearings can be used. Particularly, the provision of a ballbearing has the advantage that ball bearings have a distinctly smallerconstructional size. Further, the provision of a preferablygrease-lubricated ball bearing in this region advantageously obviatesthe need for an additional emergency bearing. In magnetic bearings, suchan emergency bearing would be positively required because, otherwise, noemergency running properties would be guaranteed in case of failure ofthe magnetic bearing.

Especially due to the relatively large bearing spacing, the forcesacting on the bearing will distribute more favorably. This is ofadvantage under the aspect of rotor dynamics. Further, possible angulardeviations of the shaft will be smaller, resulting in advantages for themechanics of the bearing. Due to the benefit of reduced angulardeviations, smaller gaps can be realized so that the efficiency of thepump can be improved and higher final pressures can be achieved.According to a particularly preferred embodiment, the inner bearingelement is fixed via a holding element. Said holding element is formedwith at least one throughflow opening. For fixing the bearing inposition, particularly for fixing the outer bearing shell in case ofrolling bearings, the holding element is preferably connected to thepump housing. With special preference, the holding element comprises aplurality of throughflow openings and particularly has a star-shapedconfiguration. The individual throughflow openings, preferably arrangedin a regular pattern and preferably having identical shapes, are withpreference configured as partial ring segments. Since, when viewed inthe conveying direction, the inner bearing element is arranged withinthe rotor element comprising the high vacuum stage, the medium will flowthrough the throughflow openings only when exiting from the high vacuumstage and respectively when entering the next stage. The conductancelosses caused by the holding element will thus be distinctly lower thanin case of an arrangement wherein such a holding element is provided inthe region of the high vacuum stage, i.e. in the gas-entrance region ofthe high vacuum stage.

According to a particularly preferred embodiment, at least two rotorelements are provided, wherein both the inner bearing element and theholding element are arranged between these two rotor elements.

According to a further particularly preferred embodiment, the innerbearing element is—along the axial direction—at least partially arrangedwithin a rotor element. Particularly, this rotor element is the rotorelement comprising the high vacuum stage. Since also in this embodimentthe rotor element as seen in flow direction is arranged before the innerbearing element, the inner bearing element is also herein arrangedbetween two rotor elements, particularly between the two fasteningregions of the rotor elements to the shaft. Due to the resultant, atleast partial covering of the inner bearing element by a part of therotor element in the axial direction, the cantilevered shaft extensioncan be made shorter. This will further improve the bearing mechanics.

Preferably, an outer bearing element is arranged in such a manner that,between the two bearing elements, a rotor element is arranged, thelatter particularly being fixedly connected to the shaft. Thus, when tworotor elements are provided, the outer bearing element is preferablyarranged outside of the rotor element forming the lowest stage. In thisarrangement, it is particularly preferred that the drive unit isarranged between the outer bearing element and the rotor element formingthe lowest vacuum stage. This has the advantage of allowing for a verylarge bearing spacing between the two bearing elements, resulting inimproved bearing mechanics.

In embodiments comprising e.g. three or more inlets and a correspondingnumber of vacuum stages, it is preferred that the outer bearing elementis arranged between two rotor elements. These will preferably be the tworotor elements forming the lowest vacuum stages, while, optionally, agiven rotor element can also form a plurality of vacuum stages.

Particularly in case of a multi-stage vacuum pump with more than twoinlets and in case of the herein preferred arrangement of the outerbearing element between two rotor elements, the outer bearing element isfixed by a holding element. Said holding element is preferably providedwith through openings and is designed corresponding to the holdingelement of the inner bearing element.

The inner bearing element is preferably designed as a rolling bearing.It is, however, also possible to provide a magnetic bearing,particularly a permanent magnetic bearing, while optionally also aretainer bearing can be provided.

According to a particularly preferred embodiment, the multi-stage vacuumpump of the invention is a vacuum pump of the multi-inlet type. Thispump is provided, apart from the main inlet, with at least oneadditional inlet. Preferably, each of said additional inlets is arrangedbetween two adjacent vacuum pumps. In usual multi-inlet vacuum pumps, apressure of 1×10⁻⁵ mbar to 1×10⁻⁹ mbar can be generated on the highvacuum side. At a first intermediate inlet, a pressure of 1×10⁻² mbar to1×10⁻⁵ mbar can be reached. In case that a second intermediate inlet isprovided, a pressure of 1×10⁻² mbar to 5×10⁻¹ mbar can be reachedthereat.

The invention will be explained in greater detail hereunder by way ofpreferred embodiments.

In the drawings, the following is shown:

FIG. 1 is a cross-sectional schematic diagram of a first embodimentcomprising two rotor elements,

FIG. 2 is a cross-sectional schematic diagram of a first embodimentcomprising three rotor elements, and

FIG. 3 is a schematic plan view of a holding element.

In the strongly simplified schematic representation of a firstembodiment of the multi-stage vacuum pump of the invention (FIG. 1), ashaft 12 is arranged in a pump housing 10. Said shaft 12 carries therotor elements 14,16 which according to the invention are separated ordetached from each other. Said rotor elements are fixedly connected toshaft 12.

In the illustrated embodiment, rotor element 14 forms a first vacuumstage 18 in which the highest vacuum is generated. The gas which is tobe conveyed is suctioned via a first inlet opening 20. In theillustrated embodiment, the first vacuum stage is a vacuum stage formedby a turbomolecular pump. A stator 22 connected to housing 10 cooperateswith said rotor 14.

The second rotor element 16 in the illustrated embodiment is arranged toform two vacuum stages 24,26. Also the second vacuum stage 24 is formedby a turbomolecular pump while, also herein, a stator 28 is connected tohousing 10. The third stage 26 is a Holweck stage wherein the helicalextension 30 is arranged in engagement with a corresponding helicalrecess. The second vacuum stage 24 will suction the medium through aninlet opening 34, and the third vacuum stage 26 will suction the mediumthrough an inlet opening 36. The suctioned medium will be conveyed byfrom all three stages 18,24,26 to the discharge opening 38.

In the illustrated embodiment, an electric motor 40 for driving saidshaft 12 is located in the region of the third stage. As providedaccording to a preferred embodiment, said electric motor 40 is arrangedto surround shaft 12. In the axial direction, said Holweck stagepreferably surrounds the electric motor 40.

Support of shaft 12 is realized by an inner bearing element 42 and anouter bearing element 44. Said inner bearing element 42 is arrangedbetween the two rotor elements 14,16. In case that the inner bearingelement 42 is a rolling bearing, an inner bearing ring is e.g. pressedonto shaft 12. An outer bearing ring is fixed via a holding element 46.Said holding element 46, shown in plan view in FIG. 3, comprises aplurality of through openings 48 formed as partial ring segments andparticularly arranged in a regular configuration, for passagetherethrough of the medium conveyed by the first stage 18.

In the illustrated embodiment, outer bearing element 44 is arrangedoutside the lowest, i.e. third stage 26. Also bearing element 44preferably is designed as a rolling bearing.

Along the axial direction 50, the inner bearing element of theillustrated embodiment is arranged within rotor element 14. For thispurpose, rotor element 14 comprises a recess 52 having a substantiallycircular cross section.

In the context of the second preferred embodiment, constructionalcomponents similar to or identical with those described above aredesignated with the same reference numerals. For ease of survey, no pumphousing is illustrated. The gas flow is indicated by arrows 54, 56, 58,60. An alternative gas flow is indicated by arrows 62, 64, 56, 58, 60.

In the embodiment shown in FIG. 2, three rotor elements 14, 66, 68 areprovided on the shaft 64. In the illustrated embodiment (FIG. 2), allrotor elements 14, 66, 68 are shown as rotor elements of molecular pumpswhile, of course, the rotor elements can also be of a different type.Further, also in this embodiment, single rotor elements can form aplurality of stages.

As in the embodiment shown in FIG. 1, the inner bearing element 42 isarranged between two rotor elements 14,66 and again is fixed orconnected to the housing by a holding element 46 (FIG. 3). In theillustrated embodiment, the drive motor 40 is arranged between the tworotor elements 14,66.

The second, i.e. outer bearing element 44 is arranged between the tworotor elements 66,68 (FIG. 2) and, in the illustrated embodiment, isfixed via a holding element 46.

Via a first flow path which is indicated by the arrows 54, 56, 58, 60,the flow will successively pass through the individual pump stagesformed by the rotor elements 14, 66, 68.

Within the second flow path which is indicated by arrows 62, 64, 56, 58,60, there is additionally performed a suctional intake of gas through afurther inlet opening in the direction marked by arrow 62. Via a bypassor connection channel (arrow 64), the gas which is sucked both in thedirection of arrow 62 and in the direction of arrow 54 will be conveyedto the next stage (rotor element 66).

The arrows 54, 62, 56 and 68 correspond to inlet openings and outletopenings, respectively. Arrow 60 corresponds to the discharge opening.

1. A multi-stage vacuum pump, comprising: a plurality of rotor elementsarranged on a common shaft in a pump housing to form a plurality of pumpstages, a drive element for driving said shaft, and an inner bearingelement arranged between two rotor elements.
 2. The multi-stage vacuumpump according to claim 1, wherein said inner bearing element is fixedvia a holding element comprising at least one throughflow opening. 3.The multi-stage vacuum pump according to claim 2, wherein said innerbearing element is connected to the pump housing via said holdingelement.
 4. The multi-stage vacuum pump according to claim 2, whereinsaid holding element is substantially round.
 5. The multi-stage vacuumpump according to claim 2, wherein said holding element is arrangedbetween two rotor elements.
 6. The multi-stage vacuum pump accordingclaim 1, wherein, along the axial direction, said inner bearing elementis arranged at least partially within a rotor element.
 7. Themulti-stage vacuum pump according to claim 1, wherein a rotor element isarranged between said inner bearing element and an outer bearingelement.
 8. The multi-stage vacuum pump according to claim 7, whereinsaid outer bearing element is arranged between two rotor elements. 9.The multi-stage vacuum pump according to claim 7, wherein said innerbearing element and/or said outer bearing element are configured asrolling bearings.
 10. The multi-stage vacuum pump according to claim 9,wherein said rolling bearing is grease-lubricated.
 11. The multi-stagevacuum pump according to claim 2, wherein the holding element defines aplurality of throughflow openings which are shaped as partial ringsegments.
 12. The multi-stage vacuum pump according to claim 7, whereinthe rotor element is tightly connected to the shaft.
 13. The multi-stagevacuum pump according to claim 8, wherein the outer bearing element isfixed via a holding element comprising throughflow openings.
 14. Themulti-stage vacuum pump according to claim 9, wherein the inner bearingelement and/or the outer bearing element are roller bearings.
 15. Amulti-stage vacuum pump comprising: a pump housing which defines atleast a first inlet and a second inlet and an outlet; a drive shaft; afirst rotor element mounted to the drive shaft and configured to drawgas into the housing through the first inlet; a second rotor mounted tothe drive shaft and configured to draw gas into the housing through thesecond inlet and to push the gas drawn into the housing through thefirst and second inlets out the outlet; a first bearing mounted to theshaft on an opposite side of the first rotor element from the firstinlet to rotatably support the shaft in the housing; a second bearingmounted to the shaft on a side of the second rotor element facing theoutlet to rotatably support the shaft in the housing; and a motormounted around the shaft between the first and second bearings to rotatethe shaft and the first and second rotor elements.